Breast cancer is one of the most common types of cancer among women and the main cause of death for middle aged women. Each year, over 180,000 new cases of invasive breast cancer are diagnosed and more than 40,000 women die from the disease in the United States. Early detection is currently the best hope for reducing the death rate of this devastating disease. Mammography typically cannot differentiate benign from malignant disease. Moreover, mammography is significantly less accurate in patients with dense glandular breasts.
Consequently, the results of X-ray mammography may show suspicious areas where no malignancy exists, with up to twenty percent of biopsied growths identified as cancerous by a mammogram being identified as malignancies. Furthermore, radiologists interpreting X-ray mammography imagery can overlook between fifteen and twenty-five percent of cancers. Finally, the very process of X-ray mammography, specifically the use of imaging plates, can cause bruising of the breasts and acts as a disincentive for women to undergo mammography screening.
Other imaging modalities have emerged to augment mammography and improve the accuracy of non-invasive breast cancer diagnosis. Ultrasound is currently used to differentiate breast masses and guide aspirations and biopsies. Magnetic resonance imaging has excellent sensitivity in demonstrating breast cancer but has a low specificity. Nuclear medicine studies have recently emerged that detect the increased metabolic rate and vascularity of breast cancers.
During the past several years, microwave imagery has formed the basis of a new, alternative detection technique for the detection of breast cancer. Whereas in X-ray mammography, high-energy ionizing radiation can be passed through the breast to a photographic plate in order to shadow potential tumors, in microwave detection, an array of antennae affixed to the breast surface can “bounce” non-ionizing microwave radiation off malignant growths whose radiation can be detected by the array. Based upon the characteristics of the “bounce”, growths can be detected much in the same way that radar can be used to detect objects at a distance.
Microwave imaging for use in breast cancer detection has been referred to as “breast tumor radar.” In a typical implementation, a computer can be coupled to an array of small antennae beaming 6 GHz pulsed microwaves. Since normal breast tissue remains largely transparent to microwave radiation, breast tumors which contain more water than normal breast tissue cause the scattering of the beamed microwaves back toward their source. The antennae can detect the scattered microwaves which can be analyzed to construct a three-dimensional image showing both the location and size of the tumor(s).
In a similar effort to microwave imaging based upon the high relative water content of the tumor, new efforts have been undertaken in the application of both ultrawideband radar technology and confocal optical microscopy. This technique can exploit the dielectric constant contrast between normal breast tissue and malignant tumors at microwave frequencies. Specifically, each element in an antenna array sequentially can illuminate an uncompressed breast with a low-power ultrawideband microwave pulse. Following the acquisition of backscattered waveforms, the array can be synthetically focused by time shifting and adding the recorded returns. A subsequent synthetic scan of the focal point permits the detection of strong scattering sites in the breast which can be identified as malignant tumors.
Microwave-induced thermoacoustic imaging is a microwave imaging technique which has demonstrated tremendous potential for tissue imaging. Thermally-induced acoustic waves can be induced in virtually any material, including biological tissue. More particularly, when tissue becomes heated, the tissue expands in size producing pressure waves, which propagate throughout the tissue in all directions. According to general principles of thermoacoustic physics, the thermally-induced acoustic signal can be compared to the manner in which the tissue is heated as well as the local tissue absorption properties within the irradiated tissue volume.
In operation, microwave-induced thermoacoustic imaging involves the use of a short-pulsed microwave beam for irradiating the biological tissue of the breast. The breast tissue can absorb the microwave energy and, responsive to the absorption of the microwave energy, the breast tissue can emanate thermoacoustic waves through thermoelastic expansion. The thermoacoustic waves can carry the information regarding the microwave energy absorption properties of the breast tissue under irradiation. The different energy absorption properties among the different types of breast tissue permit the construction of a distribution of microwave energy absorption pattern in a homogeneous acoustic medium.
For example, U.S. Pat. No. 6,567,688 to Wang discloses a microwave-induced thermoacoustic system and method for imaging biological tissue. Short microwave pulses irradiate tissue to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves are detected with an ultrasonic transducer or transducer array. Each time-domain signal from the ultrasonic transducer is converted to a one-dimensional image along the acoustic axis of the ultrasonic transducer. Scanning the system perpendicularly to the acoustic axis of the ultrasonic transducer generates multi-dimensional images in real-time without computational image reconstruction. Wang discloses use of a single microwave frequency generally in the range from 300 MHz to 3 GHz.
Although the Wang system provides fairly good quality breast images, the test conditions disclosed were highly oversimplified or not representative as compared to conditions normally present during annual breast imaging. First, the size of the test sample had been comparable to the irradiation aperture of the microwave waveguide so that the test sample was irradiated uniformly and the test sample heated effectively. Second, the samples tested comprising muscle cylinders inserted in the fat were not totally enclosed by skin or even fat. Thus, the experiment did not produce a skin effect and allowed the muscle cylinders to remain under direct irradiation of the microwave pulses.
For human breast imaging, however, the situation can be much more complicated. The human breast is much larger in size, usually has an irregular shape if not compressed, and is covered with about a two millimeter thick skin having dielectric properties significantly different from normal breast tissue. These factors make it much more difficult to heat the breast effectively. Moreover, the breast tissue is far from homogeneous. The breast is composed of lobules of lactiferous glands and ducts set in fat tissue that exit at the nipple. The propagation of the microwave-induced thermoacoustic signals in the regions containing many glandular lobules and ducts will be significantly different as compared to pure fat tissue.
Notably, the propagation paths of the microwave-induced thermoacoustic signals generated from the tumors inside the breast will change due to the refractions at the skin margin. These abnormalities in thermoacoustic wave propagation will introduce approximations into the calculation of the signal back propagation. Due to the slow acoustic wave propagation speed or short wavelength, the errors on the order of millimeters in determining the signal propagation path lengths can degrade the reconstructed image severely. For instance, experiments have shown that the thermoacoustic signals induced at the skin margin and their scatterings from the fat-glandular and fat-duct tissue boundaries are very strong, and the image quality degrades significantly at the depths greater than about forty to forty-five millimeters behind the nipple region. Therefore, the skin not only changes the microwave power deposition pattern in the breast, but also introduces strong clutter.
To enhance the thermoacoustic imaging of the human breast, several efforts have been made to improve the performance of conventional thermoacoustic imaging technology. As an example, to achieve a higher spatial resolution, an array with more transducers or more acoustic data acquisitions at a larger aperture has been considered. Alternatively, for a deeper microwave penetration, an increase of the transmitted microwave power has been an obvious consideration, and an array of multiple waveguides placed radially around the breast has also been suggested. Other reported efforts include searching for a more suitable microwave frequency and a more effective pulse duration of the microwave pulses. In summary, although microwave-induced thermoacoustic imaging has great potential for early breast cancer detection, much work needs to be done toward making microwave-induced thermoacoustic imaging a practical method for early breast cancer screening.